Mirena issues

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I got my Mirena placed in April 13. I just had my 6 week check up two days ago and the doctor couldn’t find the strings. So he had me get an ultra sound and luckily it is still in the proper place. He said that the only problem is when I want it removed it will have to be done in the operating room. He said he has had other patients that their strings went away but they did end up coming back so he’s hoping that’s what it ends up doing. Has anyone else run in to this issue? Have the strings uncoiled eventually. This makes me nervous. I’ve had the Mirena before and never had this problem.